A suspected bomb threat forced a United Airlines flight from Sydney to Los Angeles to make an emergency return to Australia. Officials say the written threat appears to have been a hoax.

United Airlines flight 840 left Sydney bound for Los Angeles with 246 passengers and crew on board.

About 90 minutes into the flight, a note was discovered indicating a bomb was on board the Boeing 747. It has been reported the note was found in a toilet. The pilot decided to abort the flight and return to Sydney.

The aircraft was searched, but no explosive device was found. A senior police official says the note appears to have been a hoax.

A United Airlines statement said, "As a precaution, the captain immediately returned to Sydney, landing without incident." Officials say the incident will be investigated further.

Australian Transport Minister John Anderson says emergency procedures were activated when the note was found.

"We would describe it as an emergency, which triggered the very real course of action right along the eastern seaboard, and indeed at airports right across Australia, as airspace was closed and everything from fire trucks to, you know, medical services and so forth were put on a very high-level alert," he said.

All crew and passengers are safe and they are expected to resume their journey on Wednesday.

A few days ago, a group claiming to be a European affiliate of the al-Qaida terror network warned it would turn Australia into "pools of blood" unless Canberra withdraws its troops from Iraq.

Australia has nearly 900 military personnel in Iraq and the government insists it will not be intimidated by such threats.